Diesel engines often employ a fuel precharge or pilot injection prior to main injection in order to reduce nitrous oxide emissions and improve fuel economy. The pilot injection is used to warm the engine cylinder and to reduce ignition delay prior to burning of the main fuel charge. In effect, the pilot injection charge helps the main injection charge burn more efficiently.
Pilot injection is typically accomplished in a diesel engine by a solenoid-actuated fuel injector. A typical solenoid-actuated fuel injector valve is illustrated in FIGS. 3 and 4. As shown, the fuel injector 10 includes a body 12 with a stepped bore 14 formed therethrough. A valve stop 16 is disposed within the stepped bore 14 and forms a chamber 18 around the head portion 20 thereof. The chamber 18 is in continuous fluid communication with the channel 22, and is in selective fluid communication with the channel 24. The control valve 26 is operative to selectively communicate and discommunicate the channel 24 from the chamber 18 by engaging or disengaging the valve seat 28.
The valve 26 is solenoid-actuated for movement between the closed position shown in FIG. 3 in which the valve surface 30 engages the seat 28, and the open position shown in FIG. 4 in which the lower surface 32 of the valve 26 engages the top surface 34 of the valve stop 16. With the valve 26 in the closed position, as shown in FIG. 3, pressurization of fuel in the flow channel 24 will cause the fuel injector to inject fuel into an engine cylinder because it is blocked from flowing into the chamber 18. However, with the valve 26 in the open position, as shown in FIG. 4, fuel may flow from the channel 24 through the chamber 18, and further through channel 22 for low pressure fuel flow between injection cycles, thereby preventing injection.
Referring to FIG. 1, a typical prior art control valve position versus cam angle graph is shown in which pilot and main injection charges are injected. For pilot injection, the control valve closes, as shown in FIG. 3, and the valve surface 30 engages against the seat 28. Due to the limited amount of force applied to the valve by the solenoid and the elastic forces involved when the valve surface 30 engages the seat 28, the valve 26 tends to bounce, as illustrated between times t.sub.a and t.sub.b in FIG. 1. Accordingly, the pilot injection charge is adversely affected. At time t.sub.b, the solenoid is de-energized so that the valve may open, and a spring (not shown) is operative to move the valve from its closed position to its normally open position shown in FIG. 4. However, when the valve reaches its open position at time t.sub.c, the lower surface 32 of the valve 26 will typically bounce against the top surface 34 of the valve stop 16 as a result of the limited force applied by the spring, and the elasticity of the contact between the valve 26 and the valve stop 16. Accordingly, as shown in FIG. 1, between times t.sub.c and t.sub.d, the valve 26 will typically rebound against the top surface 34 of the valve stop 16 numerous times. Once the bounce or rebound has stabilized, the control valve 26 will be re-closed between times td and te, as shown in FIG. 1, for main injection.
The valve bounce between times ta and tb, and between times tc and td, creates an undesirable delay between pilot injection and main injection. For example, between times tc and td, the valve must be stabilized prior to initiating re-closing for main injection, and this delay creates a large gap between pilot and main injection, which decreases the effectiveness of the pilot injection charge. This time delay can involve an 11 degree cam rotation, as illustrated in FIG. 1. Accordingly, waiting for a stable position of the valve at its closure and waiting until the rebound dies out before starting the second valve movement makes both pilot output and separation between pilot and main injection unacceptably long.
It is desirable to provide a method of reducing or eliminating valve bounce in a fuel injection system in a manner which enables reduction of separation between pilot and main injection for more efficient fuel burning.